[Diagnostic] Don't fix partial mismatch for sub-types associated with optional conversion

If mismatch detected by `repairFailures` is related to a complex
wrapped value of optional type formed from optional-to-optional
or value-to-optional conversion let's not try to fix it directly
but let `simplifyRestrictedConstraintImpl` record a top-level fix
for more context.

Resolves: rdar://problem/59703585

lib/Sema/CSDiagnostics.cpp

@@ -1884,6 +1884,12 @@ bool ContextualFailure::diagnoseAsError() {
       return true;
     }
 
+    if (isa<AssignExpr>(anchor)) {
+      emitDiagnostic(anchor->getLoc(), diag::cannot_convert_assign,
+                     getFromType(), getToType());
+      return true;
+    }
+
     return false;
   }